Ammunition Naming Conventions:
Laying the ground rules
An exercise in mixed practicality and superfluous subtitling
Ammunition is a rather broad term. So broad in fact, that before we progress any farther, we need to lay some ground rules for the rest of the article. First, the sort of ammunition we're talking about is the kind that is used to shoot bits of metal out of a tube; that is, the kind used in, for lack of a better general usage term, "guns", though this is not strictly accurate. The second ground rule is that there are exceptions. Many, many exceptions. So many, in fact, that to cover them would be like comparing irregular verbs in English; a hideous distraction from an otherwise useful topic, and interesting to only a few niche enthusiasts. For others, it suffices to be able to recognize and memorize the exceptions.
Next, let's talk about a few pieces of basic terminology; caliber refers to the diameter of the projectile, and can be measured (contrary to the strict definition of 'caliber') in either inches or millimeters, and with varying notation. The word or abbreviation for "caliber" is usually left off of metric measurements, but not always. For example, a package of projectiles might be marked ".223 Cal/5.56mm".
Lastly, let's discuss the importance of ammunition naming conventions. It's very close to the reason we have scientific names for living things, rather than relying on common names. If someone from Alaska and someone from Southern India were talking about, say, bears, there's a huge opening there for some very bad confusion, or else some wasted breath on explanations, whereas they could have used the absolutely precise terms Ursus arctos middendorffi and Ursus ursinus. The same applies when you're looking at, say, two surplus pistols, both of them designed for "9mm". Is it 9x19mm Parabelum? 9mm Makarov? 9mm Browning? In this case, the correct chambering is easy to identify, but not so in the case of many, many rounds that are sometimes dimensionally identical, but have very different pressures!
Where (C) is the caliber; (L) is the overall length ("OAL") of the cartridge, which is the total length of a loaded cartridge from the bottom of the brass to the tip of the projectile; (R?) is an optional distinction for a rimmed cartridge, often mistaken to mean "Russian"; and Designation is the "given name" of the cartridge. This is known as the "Metric Designation", "Metric Caliber", or "Metric Size" and should always be given in millimeters.
One example that highlights every possible use of this convention is 7.62х38R Nagant; It uses a 7.62mm projectile, has an OAL of 38mm, is rimmed, and is designated "Nagant" for the original weapon it was designed for.
Going back to a previous example, you see that 9x19mm Parabellum uses a 9mm diameter projectile, has an OAL of 19mm, and is designated "Parabellum". It is an extremely common round that is known sometimes simply as "9mm Para", since this is just as unambiguous as its full designation. A variant of 9mm Para is known as 9mm NATO, which is dimensionally identical to 9mm Para, and is often (but not always) interchangeable. The difference is that 9mm NATO is loaded to NATO specifications and standards, which include particular types of projectile and powder.
In fact, all NATO spec ammunition are specific standards of other ammunition, agreed on by NATO for use across all forces in times of war. Other than 9mm, the most common example of this is of course the .223 Remington/5.56mm NATO family of cartridges. The two cartridges are nearly dimensionally identical, and each can be inserted and fired in chambers marked for the other; however, the 5.56mm NATO has a slightly different case shape and operates at a higher maximum pressure than the .223 Rem standard. One should never fire a weapon with ammunition not specifically rated for it. .223 Remington brings us to our next naming convention;
Where (C) is caliber, and (Designation) is almost always the designer or manufacturer, but is sometimes an arbitrary name.
Common examples are .223 Remington, .308 Winchester, .38 Smith & Wesson, and .45 ACP (Automatic Colt Pistol). Some uncommon examples are .408 Chey-tac and .50 Vickers.
It is important to note that any ammunition can be referred to by either naming convention; in fact, most common rounds will be referred to by the second designation unless, as in the previous example of "9mm", precision is necessary in a specific context.
One glaring example of the importance of adhering to markings and terminology is the already mentioned .308 Winchester. The .308 Win is dimensionally identical to the 7.62x51mm NATO, or 7.62 NATO, cartridge; as with .223 Rem/5.56 NATO, either cartridge will fit and fire from either chamber. HOWEVER, 7.62 NATO operates at MUCH higher pressures than the standards set for .308 Win; it is absolutely unsafe to fire 7.62 NATO from a rifle not designed for it!
On the subject of mix-and-match
Many uninformed buyers, sellers, and information sources will often mix-and-match their terminology, particularly with ammunition that has commercial/military/NATO variants. It is not at all uncommon to see things like ".223 NATO" or "7.62 Win". This is a serious issue, since without further information, it is impossible to know what the cartridge might actually be. Further difficulty arises when dealing with surplus ammunition. For example, "7.62x51 Argentina" typically refers to surplus 7.62x51mm NATO-like ammo made for the Argentinian military. However, in this particular case, the ammunition has very hard primers and operates at very high pressures, usually above the upper limit for NATO cartridges; this is because they were designed for an Argentinian variant light machine gun that required the higher pressures to cycle reliably! It can be fired safely in most NATO chambered rifles, however it is NOT the same as 7.62 NATO!
With any luck, you will be able to identify ammunition based on these two naming conventions with enough certainty to look up the specifications for verification; as I mentioned before, though, there are ALWAYS exceptions, like the venerable workhorse .30-06, or .30-30, which are properly known as ".30-06 Springfield" or ".30-30 Winchester/7.62mmx51R/.30 Win CF", respectively. Coupled with patchy or complete ignorance on the part of others, there can certainly be challenges involved, but they are not insurmountable.
The name and designation of a cartridge refers to the specifications of its minimum and maximum allowable pressure and exterior dimensions. It does not generally dictate bullet weight or shape. NATO ammunition is a notable exception to this, but most commercial offerings are available in a staggering variety. One can, for example, purchase .308 Winchester ammunition with solid copper hollowpoint, jacketed softpoint, jacketed hollowpoint, full metal jacket, and lead hollowpoint bullets each in a variety of shapes like Spitzer or boattail, and in almost 30 different individual bullet weights! These are typically noted by abbreviations following the designation. For example, .308 Win 165gr JSP is a .308 Winchester with a 165 grain, jacketed soft-point bullet.
Some ammunition carries a designation of "+P", "+P+"; this indicates that the ammunition is loaded to a higher pressure than the SAAMI standard for that particular ammunition, and should only be used in a firearm capable of handling it.
Incorporating every convention and twist you've read about so far is the 9mm Luger +P 115 Grain Barnes XPB. This is a popular new offering from Cor-Bon; It is a 9mm Luger (an alternate name for the 9mm Para) loaded to a higher than standard pressure, using a 115 grain Barnes XPB bullet, which is a proprietary bullet design.
That last example is about as complicated as the average, or even advanced, shooter or enthusiast is ever likely to encounter. Beyond this are examples so esoteric or arcane that reference manuals are necessary for all but the most niche of experts.
Special care should be taken when handloading or reloading ammunition to always use and load components to the proper dimensons; Aside from the obvious problems with an incorrect type or amount of powder, some common designators have a slightly different measured caliber than is reported in the name, like the .460 Rowland, which actually uses the same .451 caliber bullets as .45 ACP.
For additional reference on terminology not detailed here, see: Firearms Terms and Terminology.